The present invention relates generally to the fabrication of integrated circuits, and more particularly to a photo-cathode image projection apparatus used for patterning semiconductor devices for use in the fabrication of integrated circuits.
In the fabrication of semiconductor integrated circuits which include numerous semiconductor devices such as very large scale integrated circuits (VLSI), extremely fine patterning is required so as to provide as many semiconductor devices as possible in a unit area. Conventionally, photolithography techniques employing ultraviolet radiation are used for this purpose. Responsive to the ultraviolet irradiation, an optical image of a desired device is focused on a photoresist deposited on a wafer or substrate through a suitable mask, and the photoresist is thus exposed to the ultraviolet irradiation in accordance with the desired pattern of the semiconductor device. Such patterning using ultraviolet light or other forms of visible and invisible light, though capable of providing a high throughput, has a basic limitation in that the minimum line thickness attainable in the patterning is limited by the relatively large wavelength of the light which is typically in the order of 4000 .ANG.. In order to achieve patterning that is finer than that which is achieved using photolithography, various techniques have been developed using other types of radiations. Among others, electron beam lithography using an electron beam as the radiation, X-ray beam lithography using an X-ray beam as the radiation, and photo-cathode image projection techniques using a photoelectron beam emitted in response to irradiation of a suitable material by an optical beam as the radiation, are being widely studied.
In the electron beam irradiation technique, an electron beam having a circular or rectangular cross-section is used for exposing the photoresist. At the time of patterning, the electron beam is deflected and moved over the surface of the wafer according to a predetermined pattern. Simultaneously, the wafer itself is moved. For focusing, shaping and deflection of the electron beam, a column system including an electromagnetic lens and an acceleration system is used. Further, a stage system is used for supporting and moving the wafer in a direction so that a desired image of pattern is written on the wafer in cooperation with the movement of the electron beam. Using a suitable acceleration voltage, a very fine image pattern can be written without using a mask. However, this technique of electron beam irradiation requires a significant time for exposure as the electron beam writes the pattern on the surface of the wafer in "one stroke" which means that the electron beam is moved over the surface of the wafer without interruption for the entire pattern. Thus, the throughput obtained by this method is relatively low and therefore this technique is not suited for mass production.
The X-ray beam lithography is a proximity printing technique in which the mask and the photoresist are separated by a minute gap, and an X-ray having a wavelength in the order of 1-10 .ANG. is used for the irradiation. This technique, though capable of providing improved resolution as compared to the conventional photolithography technique, has a problem in that a bulky X-ray generator has to be used for the X-ray source. Further, there is a problem in that the wafer, X-ray source and the mask must be aligned with extremely high precision. For this purpose, a specially designed aligner has to be used. Even so, there is a tendency for the gap between the mask and the wafer to deviate from a nominal or design value particularly when the diameter of the wafer is increased. In such a case, the gap between the mask and a wafer surface tends to vary from one position to another due to deformation of the mask or non-flat surface of the wafer. Such changes in the gap result in a blurring of the image pattern on the wafer. Further, there is a problem in that the material which can be used for the mask is limited because such a mask must absorb X-rays. Furthermore, the intensity of the X-ray beams obtained from commonly available X-ray generators i usually not sufficient for an efficient patterning operation. In other words, the throughput achieved by X-ray beam lithography is too small for mass production of integrated circuits. Of course, it is possible to consider using an intense X-ray beam such as is produced by a synchrontron orbit radiation ring (SOR) for this purpose. However, such a facility has an enormous size and is too expensive for a practical facility for fabrication of integrated circuits.
The photo-cathode image projection technique is advantageous as it provides high resolution comparable to that of electron beam lithography in combination with high throughput comparable to that of photolithography. In this technique, a material capable of emitting electrons when irradiated by a light and another material capable of emitting electrons are patterned on a mask according to the desired pattern, and the photoelectrons emitted from the mask are focused on the surface of the wafer which is coated with a photoresist. The photoelectrons emitted from the mask are accelerated and focused by magnetic and electric fields established between the mask and the wafer, and a semiconductor pattern image corresponding to the pattern formed on the mask is transferred to the photoresist covering the surface of the wafer.
Thus, a typical photo-cathode image projection apparatus comprises a mask such as the one already described, a stage for supporting the wafer coated with the photoresist, a focusing coil for focusing the photoelectrons on he wafer, a high voltage source which applies a high voltage between the mask and the stage for acceleration of the photoelectrons, and an evacuated chamber for accommodating the mask and the stage.
In such an apparatus, there is a problem in that an electrical discharge tends to occur between the mask and the wafer on the stage because of the high acceleration voltage between the mask and the wafer. When such an electrical discharge occurs, a part of the photoresist on the wafer is evaporated and scatters in the chamber. Thus, there is a substantial risk that a part of the photoresist thus scattered contaminates the mask. When this happens, the pattern on the mask is damaged and a defect is introduced into the pattern on the mask. The defect thus brought into the pattern on the mask is transferred to all of the semiconductor devices thereafter patterned on the wafer.
Further, there is another problem in that, as a result of the existence of the high acceleration voltage between the mask and the substrate, back-scattered electrons emitted from the wafer responsive to the irradiation of the wafer by the electron beam for positioning purpose, may be returned to the wafer. When such back-scattered electrons return and reach the photoresist, a part of the photoresist which should not be exposed to electrons undesirably becomes exposed. Furthermore, there is a problem in that the electrical field in the vicinity of the wafer is disturbed significantly when the surface of the wafer is not completely flat. This is due to the fact that the wafer itself is used as one of the electrodes across which the high acceleration voltage is applied. Such a disturbance in the electrical field in the vicinity of the wafer invites a significant distortion in the image of the semiconductor pattern on the wafer.